Advances in digital and wireless technologies have led to a need for faster, higher resolution signal processing systems. Analog-to-digital converters, which are key components of signal processing systems, need to be capable of handling the conversion of high-speed analog signals, such as radio frequency (RF) signals, to digital form. For example, analog-to-digital converters (ADCs) with very high sampling frequencies will be required for high-speed medical and scientific instrumentation, image processing, and wireless communication systems including software-defined radio.
The ability to sample high-speed signals will enable their direct digital signal processing. This task has proved challenging due to the need for fast and reliable ADCs. Delta-sigma modulators have recently become more practical due to improvements in technology that allow for implementation of high oversampling rates. Nonetheless, while electronic ADCs have been developed that are as fast as 18 GHz (1-bit) and 10 GHz (5-bit), the fastest commercially available electronic ADC currently known is the ADS1605/1606, a 16-bit delta-sigma ADC with a 5 MHz sampling rate, made by Texas Instruments. In order to effectively over-sample high-speed signals such as RF signals in C-band, an ADC with a sampling rate of over 100 GHz is needed.
Recently, a device having an optical switching time of 1.5 ps has been demonstrated, as discussed in Nishizawa et al., Ultrafast all optical switching by use of pulse trapping across zero-dispersion wavelength, Optics Express 11(4) 359-365 (24 Feb. 2003), incorporated herein by this reference. This and/or similar devices show promise for optical implementations of delta-sigma modulators.
Conventional modulators produce bipolar output (1, −1). A difficulty with optical implementations is handling negative values. Existing optical implementations have used the interference of light beams to overcome this problem. The use of interferometric methods, however, presents stability problems due to the laser frequency and/or phase fluctuations and component vibration.
Another disadvantage of many conventional delta-sigma modulators is that they require sample-and-hold or similar devices, which slow the oversampling rate. A further disadvantage of existing delta-sigma modulators is that they are unable to adjust the input signal range, so that when a signal of interest is beyond the input signal range, the conventional delta-sigma modulator becomes unstable and is thus unable to modulate the signal.
Accordingly, there is still a need for faster electronic and/or optical ADCs with higher sampling frequencies.